This invention relates to the playback of color picture information contained on a record such as motion picture film and, in particular, to the playback of negative film records for television programming.
Television broadcasting systems make significant use of motion picture film to complement live telecasting and prerecorded video tape sources in preparing program content.
Motion picture film sources typically comprise feature length films, commercials for advertising purposes and on-the-spot film clips of news events. Typical operating techniques for including such film sources in the program content involve the use of a telecine camera, such as the RCA TK-28, manufactured by RCA Corporation at Camden, New Jersey. Telecine cameras basically include a light source for illuminating the film, a film transport for moving the film and a television camera system to receive the illuminated film information for conversion to an electrical video signal representative of the film image information. Since the television camera portion of a telecine camera is similar to a live television camera, positive film prints made from a negative film are generally utilized, because such positive prints are color balanced; that is, equal amounts of red, green and blue colors yield white, thereby permitting direct viewing of the positive film by the camera system. However, the making of positive prints from the original negative film requires at least one extra processing step which takes time and results in a degradation of resolution as well as color saturation of the resulting positive print relative to the negative.
In view of the loss of original picture quality and the time saving possible, it would be desirable to utilize the negative film directly in the telecine cameras. A direct substitution of negative film in the telecine camera is not possible, however, unless significant changes are made in the camera system. The primary difficulty in such a direct substitution lies in the nature of negative film. As previously described, positive film is color balanced so that the transmission of light through the film is equal for red, green and blue colors, whereas, a negative film is not so balanced. A typical negative color film, for example, Eastman type 5254, has a light transmission of 71.45% red, 23.01% green and 4.898% blue.
To better understand the difficulty posed by a direct substitution of negative film in a telecine camera designed for positive film, it will be helpful to briefly review the set-up procedure for a typical telecine camera, such as the aforementioned TK-28 film camera. A film camera is usually adjusted so that when there is no film or filters in the light path from the light source to the image pickup tubes, the red, green and blue video signal outputs from the camera are of equal amplitude corresponding to white light. The variable neutral density disk commonly used in telecine cameras under these conditions is set so as to reduce the light input to the camera image pickup tubes by a factor of 10 to 1. With no film and the abovementioned disk at its low limit, the gain controls of the video amplifiers coupled to the image pickup tubes are adjusted to produce a 100% video output level. Since the maximum transmission through positive color film is approximately 50%, this allows the disk to maintain a constant video level for a film density highlight ratio of 5 to 1.
If a typical negative film, such as the Eastman type 5254 previously described, is inserted into the telecine camera, the unequal light transmission for the three primary colors will result in an unbalanced video signal output since the camera was adjusted for equal light transmission for the primary colors.
Prior art attempts to correct for this unbalance generally called for equalizing the light transmission of the negative film by inserting filters in the light path to the camera, for example, prior to the image pickup tubes, so as to reduce the red and green light transmission to match the blue light transmission of the negative film. When this method of equalization is utilized for a typical negative film, the resulting video output signal amplitude is reduced to 23% of its original 100% level with the neutral density disk at its thin limit. In order to restore the 100% level normally required for signal processing, the amplification factor of the video signal amplifiers must be increased by 1/.23 or 435%, thus raising the typical noise level in the output signal by a factor of 4.35 times. Alternatively, if optical filters are not used, the individual gain for each image pickup amplifier must be compensated so as to achieve the desired 100% video output level which results in a noise factor increase of 3.65 times. This latter solution, although an improvement over the noise factor increase of 4.35, requires a gain adjustment of the three video signal processing channels in a changeover from a feature film, which is almost always a positive print, to a commercial or news spot negative film.